5. An Overview of
Organic Reactions
Based on
McMurry’s Organic Chemistry, 6th edition, Chapter 5
©2003 Ronald Kluger
Department of Chemistry
University of Toronto
McMurry Organic Chemistry 6th edition Chapter
5 (c) 2003
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5.1 Kinds of Organic Reactions
 In general, we look at what occurs and try to learn how it
happens
 Common patterns describe the changes
 Addition reactions – two molecules combine
 Elimination reactions – one molecule splits into two
 Substitution – parts from two molecules exchange
 Rearrangement reactions – a molecule undergoes
changes in the way its atoms are connected
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5.2 How Organic Reactions Occur:
Mechanisms
 In a clock the hands move but the mechanism behind
the face is what causes the movement
 In an organic reaction, we see the transformation that
has occurred. The mechanism describes the steps
behind the changes that we can observe
 Reactions occur in defined steps that lead from
reactant to product
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Steps in Mechanisms
 We classify the types of steps in a sequence
 A step involves either the formation or breaking of a
covalent bond
 Steps can occur in individually or in combination with
other steps
 When several steps occur at the same time they are
said to be concerted
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Types of Steps in Reaction
Mechanisms
 Formation of a covalent bond
Homogenic or heterogenic
 Breaking of a covalent bond
 Homogenic or heterogenic
 Oxidation of a functional group
 Reduction of a functional group

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Homogenic Formation of a Bond
 One electron comes from each fragment
 No electronic charges are involved
 Not common in organic chemistry
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Heterogenic Formation of a Bond
 One fragment supplies two electrons
 One fragment supplies no electrons
 Combination can involve electronic charges
 Common in organic chemistry
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Homolytic Breaking of Covalent
Bonds
 Each product gets one electron from the bond
 Not common in organic chemistry
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Heterolytic Breaking of Covalent
Bonds
 Both electrons from the bond that is broken become
associated with one resulting fragment
 A common pattern in reaction mechanisms
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Indicating Steps in Mechanisms
 Curved arrows indicate breaking
and forming of bonds
 Arrowheads with a “half” head
(“fish-hook”) indicate homolytic
and homogenic steps (called
‘radical processes’)
 Arrowheads with a complete head
indicate heterolytic and
heterogenic steps (called ‘polar
processes’)
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Radicals
 Alkyl groups are abbreviate “R” for radical
Example: Methyl iodide = CH3I, Ethyl iodide =
CH3CH2I, Alkyl iodides (in general) = RI
 A “free radical” is an “R” group on its own:
 CH3 is a “free radical” or simply “radical”
 Has a single unpaired electron, shown as: CH3.
 Its valence shell is one electron short of being
complete

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5.3 Radical Reactions and How
They Occur
 Note: Polar reactions are more common
 Radicals react to complete electron octet of valence
shell
 A radical can break a bond in another molecule
and abstract a partner with an electron, giving
substitution in the original molecule
 A radical can add to an alkene to give a new
radical, causing an addition reaction
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Steps in Radical Substitution
 Three types of steps
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
Initiation – homolytic formation of two reactive
species with unpaired electrons
 Example – formation of Cl atoms form Cl2 and
light
Propagation – reaction with molecule to generate
radical
 Example - reaction of chlorine atom with
methane to give HCl and CH3.
Termination – combination of two radicals to form
a stable product: CH3. + CH3.  CH3CH3
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5.4 Polar Reactions and How They
Occur
 Molecules can contain local unsymmetrical electron
distributions due to differences in electronegativities
 This causes a partial negative charge on an atom
and a compensating partial positive charge on an
adjacent atom
 The more electronegative atom has the greater
electron density
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Electronegativity of Some
Common Elements
 The relative electronegativity is indicated
 Higher numbers indicate greater electronegativity
 Carbon bonded to a more electronegative element
has a partial positive charge (+)
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Polarizability
 Polarization is a change in electron distribution as a
response to change in electronic nature of the
surroundings
 Polarizability is the tendency to undergo polarization
 Polar reactions occur between regions of high
electron density and regions of low electron density
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Generalized Polar Reactions
 An electrophile, an electron-poor species, combines
with a nucleophile, an electron-rich species
 An electrophile is a Lewis acid
 A nucleophile is a Lewis base
 The combination is indicate with a curved arrow from
nucleophile to electrophile
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5.5 An Example of a Polar Reaction:
Addition of HBr to Ethylene
 HBr adds to the  part of C-C double bond
 The  bond is electron-rich, allowing it to function as
a nucleophile
 H-Br is electron deficient at the H since Br is much
more electronegative, making HBr an electrophile

H
Br
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Mechanism of Addition of HBr to
Ethylene
 HBr electrophile is attacked by  electrons of
ethylene (nucleophile) to form a carbocation
intermediate and bromide ion
 Bromide adds to the positive center of the
carbocation, which is an electrophile, forming a C-Br
 bond
 The result is that ethylene and HBr combine to form
bromoethane
 All polar reactions occur by combination of an
electron-rich site of a nucleophile and an electrondeficient site of an electrophile
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5.6 Using Curved Arrows in Polar
Reaction Mechanisms
 Curved arrows are a way to keep track of changes in




bonding in polar reaction
The arrows track “electron movement”
Electrons always move in pairs
Charges change during the reaction
One curved arrow corresponds to one step in a
reaction mechanism
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Rules for Using Curved Arrows
 The arrow goes from the nucleophilic reaction site to
the electrophilic reaction site
 The nucleophilic site can be neutral or negatively
charged
 The electrophilic site can be neutral or positively
charged
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5.7 Describing a Reaction: Equilibria, Rates,
and Energy Changes
 Reactions can go either forward or backward
to reach equilibrium


The multiplied concentrations of the products
divided by the multiplied concentrations of the
reactant is the equilibrium constant, Keq
Each concentration is raised to the power of
its coefficient in the balanced equation.
aA++bB
bB
aA
cC ++dD
dD
Keq = [Products]/[Reactants] = [C]c [D]d / [A]a[B]b
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Magnitudes of Equilibrium
Constants
 If the value of Keq is greater than 1, this indicates
that at equilibrium most of the material is present as
products
 If Keq is 10, then the concentration of the product
is ten times that of the reactant
 A value of Keq less than one indicates that at
equilibrium most of the material is present as the
reactant
 If Keq is 0.10, then the concentration of the
reactant is ten times that of the product
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Free Energy and Equilibrium
 The ratio of products to reactants is controlled by




their relative Gibbs free energy
This energy is released on the favored side of an
equilibrium reaction
The change in Gibbs free energy between products
and reacts is written as “DG”
If Keq > 1, energy is released to the surrounding
(exergonic reaction)
If Keq < 1, energy is absorbed from the surroundings
(endergonic reaction)
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Numeric Relationship of Keq and Free
Energy Change
 The standard free energy change at 1 atm pressure
and 298 K is DGº
 The relationship between free energy change and an
equilibrium constant is:
 DGº = - RT ln Keq where
 R = 1.987 cal/(K x mol)
 T = temperature in Kelvin
 ln = natural logarithm of Keq
 (See the example in the book for a calculation)
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Changes in Energy at Equilibrium
 Free energy changes (DGº) can be divided into


a temperature-independent part called entropy
(DSº) that measures the change in the amount of
disorder in the system
a temperature-dependent part called enthalpy
(DHº) that is associated with heat given off
(exothermic) or absorbed (endothermic)
 Overall relationship: DGº = DHº - TDSº
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5.8 Describing a Reaction: Bond
Dissociation Energies
 Bond dissociation energy (D): Heat change that
occurs when a bond is broken by homolysis
 The energy is mostly determined by the type of bond,
independent of the molecule
 The C-H bond in methane requires a net heat
input of 105 kcal/mol to be broken at 25 ºC.
 Table 5.3 lists energies for many bond types
 Changes in bonds can be used to calculate net
changes in heat
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Calculation of an Energy Change from Bond
Dissociation Energies
 Addition of Cl-Cl to CH4 (Table 5.3)
 Breaking:
C-H D = 438 kJ/mol
Cl-Cl D = 243 kJ/mol
 Making:
C-Cl D = 351 kJ/mol
H-Cl D = 432 kJ/mol
Energy of bonds broken = 438 + 243 = 681 kJ/mol
Energy of bonds formed = 351 + 432 = 783 kJ/mol
DHº = 681 – 783 kJ/mol = -102 kJ/mol
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5.9 Describing a Reaction: Energy Diagrams
and Transition States
 The highest energy
point in a reaction step
is called the transition
state
 The energy needed to
go from reactant to
transition state is the
activation energy
(DG‡)
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First Step in Addition
 In the addition of HBr
the (conceptual)
transition-state
structure for the first
step
 The  bond between
carbons begins to
break
 The C–H bond
begs to form
 The H–Br bond
begins to break
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5.10 Describing a Reaction:
Intermediates
 If a reaction occurs in more than one step, it must
involve species that are neither the reactant nor the
final product
 These are called reaction intermediates or simply
“intermediates”
 Each step has its own free energy of activation
 The complete diagram for the reaction shows the free
energy changes associated with an intermediate
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Formation of a Carbocation
Intermediate
 HBr, a Lewis acid, adds to
the  bond
 This produces an
intermediate with a positive
charge on carbon - a
carbocation
 This is ready to react with
bromide
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Carbocation Intermediate Reactions
with Anion
 Bromide ion adds an
electron pair to the
carbocation
 An alkyl halide produced
 The carbocation is a
reactive intermediate
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Reaction Diagram for Addition of HBr
to Ethylene
 Two separate steps,
each with a own
transition state
 Energy minimum
between the steps
belongs to the
carbocation reaction
intermediate.
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Biological Reactions
 Reactions in living organisms follow reaction
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diagrams too
They take place in very controlled conditions
They are promoted by catalysts that lower the
activation barrier
The catalysts are usually proteins, called enzymes
Enzymes provide an alternative mechanism that is
compatible with the conditions of life
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